1. Field of the Invention
The present invention relates to an information recording apparatus which magnetically records information which consists of a plurality of bits by a PPM recording method.
2. Related Background Art
The PPM (pulse position modulation) recording method is a per se known method of magnetically recording data upon a magnetic recording medium. FIG. 9 is a waveform diagram illustrating this PPM recording method.
With the PPM recording method, the period of the signal is always kept fixed, and the signal levels "0" and "1" are coded by the position of the trailing edge of the signal waveform. In concrete terms, if the trailing edge of the signal waveform occurs between the start point of the period of the signal and its midpoint this is taken as encoding a "0", while if the trailing edge of the signal waveform occurs between the midpoint of the period of the signal and its end point this is taken as encoding a "1". For example, referring to FIG. 9, when the trailing edge of the signal waveform occurs at a position approximately one quarter period from the head of the signal period this is taken as encoding a "0" (in this case, called a PPM signal of 25% bit location), while when the trailing edge of the signal waveform occurs at a position approximately three quarters period from the head of the signal period this is taken as encoding a "1" (in this case, called a PPM signal of 75% bit location).
The above method enables data to be transmitted correctly even if the trailing edge position in the signal period is somewhat shifted due to a delay over the data line or other cause. When data of "0" is to be transmitted in FIG. 9, it is recognized as data of "0" so long as the signal waveform drops before its half period, even if the data that should drop at one quarter period from the head of the signal period is lagged to actually drop behind the quarter period due to a delay over the data line or other cause. As a result, an error in transmission of data is less likely to occur.
Meanwhile, there are known cameras of the type magnetically recording photographic information, such as the date and exposure data, on a film, and there is also a possibility of taking advantage of the above described PPM recording method for such magnetic recording.
A photographic information output circuit in the camera outputs parallel data for information, such as the date and exposure data. Consequently, the parallel data has to be converted into serial data first and then the serial data has to be converted into a PPM signal in the camera. A conventional camera converts the parallel data into the serial data and the serial data into the PPM signal by software process of a CPU.
However, the CPU takes much time to convert all photographic information into the PPM signals, because the CPU converts the parallel data which indicates the photographic information into the PPM signal one bit by one bit. On the other hand, the CPU has to process many functions, such as an exposure process and a film feeding process, in addition to the above described converting process. Consequently, if the CPU takes comparatively long time for the above described converting process, it influences other functions. For example, the speed of consecutive shots becomes slow.